The present disclosure relates to a piezoelectric device equipped with a drawing electrode electrically coupled to an exciting electrode on a piezoelectric element and with a substrate bonded to this drawing electrode with a metallic brazing material.
FIG. 7 is a schematic sectional diagram of an existing piezoelectric device 1 (see, e.g., Unexamined Patent Publication No. 2004-328442).
This piezoelectric device is composed of three stacked crystal substrates 2, 3, 4.
The crystal substrate 2 is disposed between the crystal substrates 3 and 4. Upper and lower planes of a central portion 2a of the crystal substrate 2 are recessed and shaped into so-called inverse mesas, thereby forming inner spaces Ss above and under the central portion 2a. Exciting electrodes 5a, 5b are provided on the upper and lower planes of the central portion 2a of the crystal substrate 2 to actively vibrate the central portion. The exciting electrode 5b on the lower plane is electrically coupled to a mounting terminal portion 7 via a drawing electrode 6b. The exciting electrode 5a on the upper plane is drawn via a drawing electrode 6a up to an inner plane of a through hole 9 and electrically coupled to the mounting terminal portion 7.
The crystal substrate 3 has through holes 8, 8 at positions where the drawing electrodes 6a, 6b are provided. Filled in these through holes 8, 8 are conductive materials 10, connected to the drawing electrodes 6a, 6b. These conductive materials 10, 10 are slightly protruding from a lower plane of the crystal substrate 3, and the protruding portions make the mounting terminal portions 7, 7.
Further, the crystal substrates 3 and 4 are bonded and fixed using a metallic brazing material 11 such as gold/tin, interposing the crystal substrate 2 therebetween. Since it is necessary to seal the inner space S, the brazing material 11 is circumferentially provided and then heated and melted to bond the three crystal substrates 2, 3, 4 together.
Unexamined Patent Publication No. 2004-328442
Such a piezoelectric device 1 is structured so that the drawing electrodes 6a, 6b come in contact with the brazing material 11. Thus, when the brazing material 11 is heated and melted in order to bond the crystal materials 2, 3, 4 together, the metallic brazing material 11 such as gold/tin spreads to the drawing electrodes 6a, 6b. If the spreading extends to the exciting electrodes 5a, 5b, the brazing material 11 attaches to the exciting electrodes 5a, 5b and increases the weight of the central portion 2a. This results in fluctuation of the crystal impedance level (hereunder referred to as “IC level”) and the frequency, making it impossible to obtain desired characteristics of the piezoelectric device.
Also, when the brazing material 11 is heated and melted so as to bond the crystal substrates 2, 3, 4, so-called solder eating occurs, in which the drawing electrodes 6a, 6b are absorbed in the metallic brazing material such as gold/tin. If such solder eating spreads to the exciting electrodes 5a, 5b, portions of the exciting electrodes 5a, 5b are absorbed, reducing the weight of the central portion 2a. This results in fluctuation of the CI level and the frequency, making it impossible to obtain the desired characteristics of the piezoelectric device.
As herein described, there is a problem that the CI level and the frequency fluctuate due to the spreading of brazing material 11 or the solder eating and that it becomes impossible to obtain the desired characteristics of the piezoelectric device.